The mountain ranges in the Basin and Range Province usually trend north-south or northwest-southeast and consist of uplifted blocks of igneous, sedimentary, and metamorphic rocks. Most of the mountain ranges in Arizona's Basin and Range Province have igneous cores. Many of these are composed of Post-Paleozoic volcanic and intrusive rocks. Four pulses of igneous activity are recognized in the Basin and Range Province of southern Arizona. The earliest occurred during the middle of the Mesozoic Era. By early-Cretaceous times vast amounts of andesitic lavas, tuffs, and agglomerates had accumulated as a result of this volcanism. The second pulse occurred during Laramide times and produced rhyolitic to andesitic dikes and several granitic plutons. Most of the mineralization in southern Arizona is associated with Laramide igneous activity. The third pulse occurred during the mid-Tertiary and produced abundant rhyolites and andesites, with some minor basalts. The final pulse occurred during fairly recent Quaternary times and produced mostly basalt lava flows.
The Tumacacori Mountains form the northeast boundary of Arizona's "silver country". They consist almost entirely of Tertiary-age rhyolites. A small exposure of Laramide granite and some minor exposures of Pennsylvanian/Permian sedimentary rocks and early-Cretaceous andesite occur on the northern spur of the range. A large exposure of early-Cretaceous andesite crops out along the western flank of the range. The Tumacacoris contain no major mines and are nearly devoid of mineral deposits.
The Atascosa Mountains lie directly south of the Tumacacori Mountains, and form part of the southeast boundary of southern Arizona's famous "silver country". They share the boundary with the adjoining Pajarito Mountains which lie immediately to the south. The Pajaritos parallel the Mexican border and therefore form the southeast corner of the area of interest. The two ranges harbor no major mines and are weakly mineralized, at best. A large fault runs north-south and lengthwise through the Atascosa Mountains; the west side is downthrown relative to the east side. The Atascosas are volcanic in nature, being composed mostly of Tertiary rhyolites and older early-Cretaceous andesites. The older andesites occur on the eastern flank of the range - this is the upthrown portion of the mountain range. The Pajarito Mountains consist of a wedge of Cretaceous sedimentary rocks in the western part of the range, and early-Cretaceous andesites in the eastern part. A thin exposure of Laramide granite crops out along the northern and eastern margins of the range.
The San Luis mountain chain forms the western boundary of the area of interest. The San Luis Mountains consist mostly of early-Cretaceous "red beds" in the northern half of the range and Laramide granites in the southern half.
The Las Guijas Mountains lie just north of the San Luis range. They form the northwest boundary of southern Arizona's silver-mining country. The Las Guijas Mountains are richly mineralized and are host to many mines and prospect pits. Laramide granite forms the core of the range, while Jurassic-age rhyolitic to andesitic lava flows occur along the southwest flank of the range. Early-Cretaceous "red beds" crop out along the northern flank of the range.
The famous Cerro Colorado Mountains are also richly mineralized and contain many mines. The Cerro Colorados are a small, highly eroded volcanic range consisting of early-Cretaceous andesites in the southwest part of the range, and younger Tertiary rhyolites and andesites in the northeast portion of the range. A small Laramide diorite stock occurs in the southwest part of the range. Most of the mines and prospect pits in the Cerro Colorados occur in the southwest portion of the range.
Cobre Ridge lies in the south-central part of the area of interest. The mining towns of Oro Blanco and Ruby are located just east of Cobre Ridge. The area is richly mineralized and is home to many mines and prospect pits. The ridge consists of Jurassic-age rhyolites and andesites intruded by numerous northwest-southeast trending felsic dikes. The dikes are Laramide in age.
Quaternary alluvium blankets the pediments and comprises the fans, terraces, and stream deposits that drape the surrounding mountain ranges.
Prospectors may want to concentrate on the rugged country surrounding the town of Ruby, but especially the area south and southwest of the town. The dry creeks and arroyos that roughly correspond to the old Carrizo Creek drainage may also be worth a look. One account of the story mentions a clay hardpan or caliche deposit in association with the silver. Prospectors may want to focus on these depositional features in their search for the lost field of silver. In any case, a metal-detector may prove to be a useful tool in this area.